This invention is concerned with a suction-restricted piston pump, in particular, a radial piston pump. Piston pumps are frequently actuated by varying speed drive units, such as internal combustion engines. However, the required flow rate, frequently, is available at a low drive speed, and does not increase with a rising drive speed.
To adapt the supply characteristic to this requirement, DE-AS 20 61 960 describes a radial piston pump having cylinders radially disposed in a housing approximately lying within a plane, and spring-loaded pistons actuated by an eccentric, wherein the pumped fluid is drawn in through grooves disposed circumferentially of the eccentric. The fluid is pumped through hollow pistons and through at least one check valve in the housing. The pistons are configured as throttles in that a restriction plate is respectively provided between a shoulder in the eccentric-sided end of the pistons and the restoring springs. This configuration insures that, with a rising speed, an increasing resistance is created on the pump fluid on the suction side so that the volumetric delivery, after the pump reaches a predetermined speed, no longer rises linearly with speed but rather reaches a maximum value which is almost independent of further pump speed increases.
Hydraulic oils are only slightly compressible. The pressures developing during movement of the piston can, therefore, become very high, resulting in overloading of the pump, or causing the pump to be stalled.
To overcome this problem, control slots have been provided both on the intake side and on the pressure side, which extend over a major area along the direction of movement of the piston bore to thereby steady both the suction and the pumping operation. Pumps of this type, generally, are satisfactory in operation.
However, substantial problems will arise once it is attempted to have such pumps provided with control slots employed in suction-restricted operations. As long as the operation is still in a low speed range, the cylinders are completely filled with pressure fluid, as in pumps with no suction-restricted operation, and hence, a pump of this type operates as a pump with no suction restriction. However, once a critical speed is exceeded, the respective pumping chambers, during the suction process, no longer are completely filled with hydraulic fluid so that a very low pressure or vacuum prevails within the pressure chamber upon commencement of the compression operation of the piston. If a vacuum forming pumping chamber of this type has access to the pressure of the pressure-sided control slot under the output pressure of the pump, the pumping chamber is abruptly filled with pressure fluid which, in the continued rotary movement of the pumping chamber, is compressed in the usual manner and prior to reaching the end of the pressure-sided control slot, is again forced out by the piston.
The operations described hereinbefore result in substantial noise, which is very disadvantageous, especially so if the working environment of the pump is quiet. This applies, for example, to modern automotive vehicles increasing levels of noise suppression. Incidentally, the movement of the pressure fluid from the pressure channel, through the pressure-sided control slot, into the pumping chamber and back involves a substantial loss in output, thereby unnecessarily loading the drive unit of the pump.
The afore-described operations, in analogy, also apply, in modified form, to the suction side so that also in this respect measures will have to be taken to insure a noise reduction and to reduce losses in output. However, in this respect, it will have to be noted that the vacuum in the pumping chamber developing on the suction side can be more easily controlled than the incompressible hydraulic fluid on the pressure side. It is, therefore, quite possible, by reducing the length of the suction-sided slot, to attain a throttling effect, thus foregoing the provision of a separate throttle, thereby insuring an enhanced output and a reduced noise development. The length of the suction-sided slot is only a fraction of the length of the pressure-sided slot. Optionally, the suction-sided slot can be eliminated altogether.
Thus, a disadvantage involved with the state-of-the-art types of pumps resides in the non-uniform delivery occurring during operation on an effective restriction of the suction flow, creating steep pressure drops, during opening of the pressure valves, and mechanical noise during opening and closing of the pressure valves. Accordingly, conventional pumps are relatively noisy in operation for which reason they are not suitable for a variety of end-use applications, such as in private passenger cars.
A radial piston pump of the afore-mentioned type is also known from DE 37 00 573 A1. The rotor of the prior known radial piston pump is rotatably disposed on a control pin which is provided, in the plane of the piston bores, with two control slots of a large cross-section compared to the piston bores, and being substantially uniform throughout the length thereof. To reduce non-uniformities of the liquid flow, in the conventional radial piston pump, a restricting communication leads from the high-pressure control slot, to a pressure chamber formed in the control pin. A passageway emerges from the pressure chamber which terminates on the web and approximately at the outer dead center between the low pressure control slot and the high pressure control slot, based on the direction of rotation of the rotor and, periodically, is in communication with the pumping chambers in the piston bores. This is to insure an improved switch-over from the low pressure side to the high pressure side at the dead center. However, that measure is not suitable to avoid, in a control of the rating, through restriction of the suction flow, pressure pulsations when the piston bore pumping chamber is only partly filled during the suction stroke passes to the high pressure level of the control slot of the pressure side. Piston pumps of that type, hitherto, have not been used with a suction-restricted control on the intake side.
The invention, therefore, has as its object to reduce noise and power requirements of the above described type of pump by simple means.